1. Field of the Invention
The invention relates to the continuous casting of tubular shapes, and is more particularly directed to the continuous casting of tubular shapes directly from the molten material using a fluid-cooled mold and centrifugal deposition of the material.
2. Prior Art
The advantages of producing tubular shapes of metals and alloys directly from the molten material by casting, rather than by shaping solid metals into tubular shapes, are so apparent that many techniques have been developed for casting these shapes. Of the many types of known tube casting processes, two general classes are of interest with respect to the present invention: the continuous centrifugal casting processes and the incremental deposition processes.
The continuous centrifugal casting processes achieve their objective by pouring liquid metal into a rotating mold which is externally cooled to remove the latent heat of fusion and so solidify the molten metal within the mold. These processes operate in a continuous manner by pulling the solidified tubular shape from the mold at a constant rate. The solidified tube or pipe may be rotated at the same speed as the mold or, for reasons discussed below, at a different speed. These processes generally provide for cooling of the tube after it exits from the mold since extraction of heat through the mold is inefficient and the tube is still very hot when it exits from the mold.
Examples of continuous centrifugal casting are disclosed in the following U.S. Pat. Nos.:
______________________________________ Patent Patentee ______________________________________ U.S. 2,752,648 Robert U.S. 3,605,859 Leghorn U.S. 3,616,842 Leghorn U.S. 3,771,587 Poran Brit. 15,912 Lane et al. Brit. 22,708 Maxim et al. ______________________________________
The success of tube formation by continuous centrifugal casting has been limited due to problems created by high friction between the mold and the solidifying tube. In all continuous casting processes these frictional forces cause problems because the solidifying metal must be kept in close contact with the mold to permit heat removal through the mold as the partially solidified strand is drawn from it. The problems created by this friction are particularly severe in the centrifugal processes because rotation of the tube at speeds high enough to hold the molten metal in the desired shape until it solidifies produces very high forces which press the tube into intimate contact with the mold.
Different solutions have been proposed to overcome this problem, with limited degrees of success. Such solutions have included the use of slippery mold linings or a layer of higher-density material as a continuous lubricating film (Maxim et al.); use of pressurized gas in combination with a continuous lubricating film (Leghorn); injection of hydrocarbon lubricants (Poran); and creation of relative motion, rotationally or axially, between the mold and the tube (Robert, Poran). Each proposed solution has its shortcomings. Thus, with these continuing problems, even after many years of development efforts the various continuous centrifugal casting processes are still not economically attractive operations.
The Maxim patent, which discloses the use of a continuous film of molten metal of greater density than the tube material, introduces the higher-density metal into the leading end of the mold, together with or close to the introduction of the tube-forming metal. The centrifgual force causes the two immiscible metals to partition, with the denser metal forming a continuous lubricating film on the inner surface of the mold. This approach is attractive in principle, but is of limited utility because of several shortcomings.
First, few immiscible metal pairs have the correct ratios of melting points and densities to make the process feasible, the only metals of real commercial potential being iron and some of its alloys, paired with lead. The process is further limited because the temperature of the lubricating metal must be kept above its melting point so that the efficiency of heat removal is very poor. Another problem of this process is that the lubricating metal must be continuously cycled through the system because it spills out the open end of the mold, again leading to process inefficiencies. Efforts to minimize this overflow by limiting the exit size of the mold led to startup problems because of the varying diameter of the tube during this stage.
The second general class of processes of interest to the present invention are those which employ incremental deposition techniques to build up a body of the desired thickness, and may be considered within two subclasses: those which build up thin ribbons or sheets of metal into a thicker layer, and those which spray deposit molten droplets onto a form to build up a deposit of the required thickness.
Processes in which thin ribbons or sheets are built up incrementally do not include processes which produce tubes, but their teachings relate to the present invention. These processes are recent developments, having been inspired by recent interest in rapid-solidification processing of metals. Examples of this art include U.S. Pat. Nos.:
______________________________________ Patent Patentee ______________________________________ U.S. 3,971,123 Olsson U.S. 4,326,579 Pond et al. U.S. 4,428,416 Shimanuki et al. ______________________________________
Due to problems associated with these techniques, principally related to thermal contraction, efforts to build up sheet materials by these types of incremental deposition processes have thus far failed to lead to the commercial production of rapidly-solidified strips more than a few thousandths of an inch thick.
The latter subclass of incremental deposition processes can produce thicker sheets as well as tubular shapes, but these processes also exhibit important limitations. Illustrative of the many endeavors in this area are U.S. Pat. Nos.:
______________________________________ Patent Patentee ______________________________________ U.S. 2,864,137 Brennan U.S. 3,670,400 Singer U.S. 4,512,384 Sendzimir Brit. 1,517,283 Singer ______________________________________
The Singer patents describe several processes by which molten metal, atomized into droplets by a gas or by centrifugal means, is sprayed onto a cooled substrate to build up a bulk material, including the formation of tubes by spraying onto moving mandrels which must be removed after the tubes are formed and cut into sections the length of the mandrels, and the formation of large-diameter tubes by spraying the droplets radially outward onto the inner surface of a reciprocating cylindrical mold. The tubes must be subsequently hot-worked.
The most serious problem inherent to these spray deposition processes is that the deposits are inherently porous. This is true because when the molten metal droplets impact upon the cool substrate and upon the previously deposited metal they splash out into irregularly-shaped "splats" without completely wetting the perimeters of previously deposited droplets. These pores are very difficult to eliminate because atomized droplets exhibit a wide range of particle sizes, and droplets of different sizes freeze in different ways when they strike cool surfaces. This problem is generally dealt with by consolidating the deposits after they are formed, most often by hot rolling the product. This approach is undesirable because oxidation of the void surfaces or the presence of included gases often leads to problems in generating high integrity materials.
Attempts are sometimes made to minimize the formation of voids by adjusting the process conditions such that each new impacting droplet strikes the accruing surface just before the last deposited droplet solidifies so that the new droplet fully wets the surface and leaves no voids. This solution is not entirely satisfactory. A major shortcoming of this approach is that the rate of cooling diminishes as the deposit increases in thickness, with the result that it is difficult to control the process variables so as to continuously maintain a thin layer of liquid metal at the product surface. This problem is aggravated by the fact that the diverse atomization processes all produce particles of a wide range of sizes, and each of these sizes solidifies at a different rate. Because of this effect large particles will still be molten, perhaps having a temperature near the original superheat temperature, when they strike the product, while the finest particles will be fully solidified. This makes it very difficult to generate uniform, pore-free structures.
The decreasing rate of cooling during the buildup of thick layers has undesirable effects in addition to that of altering the nature of pore formation. The initially-deposited material, which cools most rapidly, has very fine microstructural features, with minimal partitioning of alloy constituents. The later-deposited material has coarser microstructural features, more segregation and, as a consequence, less desirable properties. Efforts made to overcome this problem include periodically interrupting the deposition process or by moving either the atomizer or the product in a reciprocating fashion, but these means are not fully satisfactory as they can lead to banded structures.
Further serious problems, associated with the degree of bonding between the spray deposit and the mold or substrate material upon which the metal is deposited, are not productively addressed in the above-cited patents. In order to achieve good heat transfer characteristics between the deposit and the substrate, it is desirable that the deposit be well bonded to the substrate. On the other hand, in order to separate the deposit easily when it has reached the required thickness, it is desirable that the bond be weak. As a consequence, compromises must be made, and these compromises generally lead to separation of the deposit before it has reached full thickness. Material deposited after the separation cools even more slowly. This problem is particularly acute in the continuous tube forming processes, in which the deposit can not be bent to separate it from the mold. Thus, when the deposited tube is being pulled from the mold, portions of it tend to stick and, because the hot and porous metal has little strength, these portions can break off and be left behind in the mold.
An important characteristic of atomized metal sprays contributes to this last-mentioned problem. Virtually all atomized sprays spread out in directions normal to their nominal flight path. This means that when the spray is directed from an atomizer near the center of the tube outwardly toward its wall, the droplets are deposited over a range of positions along its wall. Near the outer reaches of this spray pattern the rate of deposition is low compared to the rate of deposition in the center portion. In this "overspray region" furthest from the open end of the mold the rate of buildup is so slow that some of the material is, of necessity, left behind as the tube is pulled from the mold.
Singer (U.S. Pat. No. 3,670,400) and Sendzimir (U.S. Pat. No. 4,512,384) address this problem by providing for reciprocation of the mold, the same technique used in continuous casting processes. As in those processes, when the mold is advanced past the less rapidly moving deposit, some material is dragged off the mold by the deposit and builds upon the end of the deposit. In continuous casting processes, accretions such as these are welded to the deposit as the surrounding melt solidifies, or, if they are deflected away from the mold and into the melt, they can be remelted. The net effect is that their accretion does not seriously mar the integrity of the casting. With the spray deposition processes, however, there is no natural mechanism present to fuse the accretion smoothly to the deposit, or to remove them if they assume awkward attitudes. If an accreted mass of some size shadows part of the deposit surface from the spray, then a large void will be left in the tube wall. Similarly, protrusions on the inner surface produced by the accretion of large fragments will be subsequently built up at least as fast as the surrounding material, so they will result in the formation of irregular bumps or protrusions on the inner wall of the tube.
All of the shortcomings of the prior art, as discussed so far, are addressed in the present invention. This invention also makes possible the formation of tubular products of one metal lined with a second metal, and while such products have been produced in short lengths by techniques such as chemical, electrochemical, and vapor deposition means, as well as by plasma spray deposition to the inside of preformed pipes, none of the processes described above have been used to make such products. The advantages of producing such products by continuous casting techniques, as opposed to the deposition techniques just referred to, is that products so formed are more easily welded without destroying the coating in the vicinity of a joint. Composite tubes or pipes made by continuous casting can have inner or outer layers of corrosion resisting materials which are of substantial thickness and which are well bonded to the base metal making up the major portion of the pipe's volume. These protective layers can be welded before or after the base metal is welded. The joints so formed can have all the strength and corrosion resistance the composite pipe exhibits before it is welded. Preformed pipes which are coated by the known processes can not be satisfactorily joined by welding because the coatings are too thin to be separately welded. Furthermore, welding of the base metal destroys these coatings in the vicinity of the joint, with the result that the joint must then be recoated, a process which is generally impractical.
Composite pipes made up of two different metals are made routinely in industry by first forming a composite billet consisting of a thick-walled cylinder of one metal closely fitted inside a thick-wall cylinder made of the other metal. These two cylinders are joined at their ends by welding and then this billet is reduced to a pipe by standard metal working processes. Pipes made by this process can be satisfactorily welded, but they are not widely employed because they are costly to produce.